Ultrafast Tunable Terahertz-to-Visible Light Conversion through Thermal Radiation from Graphene Metamaterials

Several technologies, including photodetection, imaging, and data communication, could greatly benefit from the availability of fast and controllable conversion of terahertz (THz) light to visible light. Here, we demonstrate that the exceptional properties and dynamics of electronic heat in graphene allow for a THz-to-visible conversion, which is switchable at a sub-nanosecond time scale. We show a tunable on/off ratio of more than 30 for the emitted visible light, achieved through electrical gating using a gate voltage on the order of 1 V. We also demonstrate that a grating-graphene metamaterial leads to an increase in THz-induced emitted power in the visible range by 2 orders of magnitude. The experimental results are in agreement with a thermodynamic model that describes blackbody radiation from the electron system heated through intraband Drude absorption of THz light. These results provide a promising route toward novel functionalities of optoelectronic technologies in the THz regime.

with the graphene sheet, and gate electrodes (G) that are not touching the graphene. The electrodes are wire-bonded to metallic leads mounted on a sample holder for electrical control. The final fabrication step consisted of applying a transparent polymer electrolyte top gate, consisting of PEO and LiClO4 with 8:1 weight ratio in a solution of methanol, that was uniformly deposited on top of the graphene and the two gate electrodes. The graphene resistance was measured using the S and D electrodes, while the Fermi energy was controlled by changing the voltage on the G electrodes. Sample B is a highly doped intercalated fewlayer graphene sample, dubbed "graphexeter", which we obtain following references [S1, S2].
Briefly, we transferred a few-layer graphene film, grown by CVD on nickel, onto an Infrasil quartz substrate followed by intercalation with ferric chloride (FeCl3). The intercalation process took place using the vapor transport method in a two-zone furnace. We placed anhydrous FeCl3 powder and the graphene sample at different positions inside a glass tube and evacuated the tube down to a pressure of 10 −6 mbar to reduce contamination by water molecules. The sample and the FeCl3 were heated for 12 hours at 360°C and 315°C respectively, which allows for the sublimation of the FeCl3 molecules and their intercalation into the few-layer graphene film. Sample C, the metamaterial-grating graphene sample, was also produced by transferring CVD-grown monolayer graphene (approximately 1x1 cm 2 area) onto an Infrasil quartz substrate. We then applied atomic layer deposition of 2 nm of Al2O3 and then used optical lithography and thermal evaporation of 50 nm of gold with titanium in order to create the metallic stripes. These have a width on the order of 20 microns and are separated by a gap of ~2 microns.
The THz electric field time trace was measured by free-space electro-optic sampling in 2 mm thick (110) ZnTe. The THz beam waist (approximately 2 mm FWHM) was detected by a pyroelectric array camera at normal incidence. Knowing the THz beam diameter, pulse energy and electric field time-trace the THz intensity was determined (Supplementary Note 4). THz power was varied using two subsequent THz free-space wire-grid polarizers (the second one determines the transmitted THz polarization and its orientation was fixed). The THz-induced PL was collected by a lens in the specular reflection direction within 1 inch aperture at 5 inch distance from the sample and then detected using a time-correlated single photon counter. The spectral information was obtained using an additional built-in monochromator.

Supplementary Note 3, Calculations of electron temperature:
In order to compare the electron temperatures that we obtained from describing the emission spectra by black-body radiation, we also calculated the expected electron temperatures based on the incident power, THz absorption and heat capacity of the graphene sheet using the following procedure. We used an incident THz fluence of Fin = 13.5 μJ/cm 2 and a THz absorption A determined by the sheet conductivity σ via the thin film approximation: Here A is absorption, Z0 is the free-space impedance, and ns is the refractive index of the substrate (1.5 for our quartz samples). For gate-controlled graphene sample A, we used the gate-dependent resistance (R) measurements (see Fig. 1c in the main text) and subtracted an estimated contact resistance of Rc ~ 1.3 kΩ, in order to obtain the sheet conductivity = 1 − . For samples B and C, we obtained the sheet conductivity using = , where n is the carrier density and is the charge mobility. For the intercalated few-layer graphene Sample B, we used n = 2.2 . 10 13 /cm 2 and = 700 cm 2 /Vs (from Raman measurements, see Supplement). For grating-graphene metamaterial sample C, we used n = 4 . 10 12 /cm 2 and = 2000 cm 2 /Vs (based on Deinert et al, [S4]). We then calculated the absorbed THz fluence as Fabs = A . Fin for Samples A and B, while for Sample C we multiplied this by the absorption enhancement factor of 2.5. Finally, we calculated the electron temperature using: , where T0 is the initial temperature (300 K) and = 21.636• 3 (ℏ ) 2 , which is valid in the regime Te > TF = EF/kB [S5]. For the simulations we use an incident fluence of 13.5 µJ/cm 2 , which is lower than the experimentally estimated fluence.
This means that the actual spot size might be a bit larger, for example due to the 45 degree incidence. It could also mean that not all incident power is converted perfectly into electronic heat. Importantly, this lower fluence does not affect any of the functional dependences that we studied, namely how the electron temperature scales with Fermi energy (via the gate voltage) and on incident power.

Supplementary Note 4, THz fluence and intensity:
Terahertz electric field distribution.
The THz pulse energy (W) and focal beam waist at full width half maximum (2·a) are 8µJ and 2mm, respectively. To calculate the fluence (F) and intensity (I) we accounted the Gaussian spatial distribution of the THz beam and measured THz electric field time-domain distribution by means of electrooptic sampling. The pulse energy can be described as where F0 is a fluence value in the center of the THz beam: Knowing the THz fluence and electric field distribution (Ei) we can estimate the THz intensity (Ii) in absolute units: Where i is a point number on the time trace, Nthe total number of points, Imax and Emax are the maximal intensity and electric field values and ∆ is a time-step value. Then, for Imax we get the following value: